The present invention relates to a method and apparatus for improving the quality of metal castings. More particularly, the present invention relates to a method and apparatus for controlling the heat extraction of molten metal being cast in a continuous caster.
The continuous casting of molten metal into ribbons, strips, sheets and slabs has been achieved through a number of processes, including, roll casting, belt casting and block casting. As used herein, the term xe2x80x9cmetalxe2x80x9d refers to any number of metals and their alloys, including without limitation, iron, aluminum, titanium, nickel, zinc, copper, brass and steel. In general, continuous casters comprise a continuously moving mold to which molten metal is supplied. The term xe2x80x9cmold,xe2x80x9d as used herein, includes any system of rollers, belts or blocks which are used to define a casting region in a continuous caster. Heat transfer from the molten metal to the mold at the metal/mold interface results in solidification of the metal. Physical characteristics of the cast metal, such as thickness, can be determined during casting by, among other things, the contact time of the metal with the mold surface and the temperature differential across the metal/mold interface.
For example, in a typical continuous block casting process used in the production of aluminum strip, such as that described in U.S. Pat. No. 3,570,586, by Lauener, assigned to Lauener Engineering Ltd., the block caster mold includes two counter-rotating, endless block chains. The block chains are comprised of a number of chilling blocks, referred to herein as xe2x80x9cblocks,xe2x80x9d which have been linked together. Each block chain is formed into an oval xe2x80x9ccastingxe2x80x9d loop by placement on a track. As the blocks travel through the casting loop, the blocks in each chain are forced together in the casting region to form a flat plane, continuous mold. The block caster can further comprise a side dam system for preventing the metal being cast from escaping the mold by travelling in a direction transverse to the casting direction. In other embodiments, the blocks themselves may be designed with ridges to prevent molten metal from escaping the mold cavity. Heat transfer from the molten metal to the blocks results in solidification of the metal.
It is desirable when continuously casting molten metal to be able to control the quality of the metal being cast. The term xe2x80x9cquality,xe2x80x9d as used herein, when referring to the metal being cast, refers to measurable characteristics of a metal cast, including, but not limited to, the number of surface imperfections in the cast, the microstructure of the cast, or the width and thickness of the cast. One method for controlling the quality of the cast in a continuous caster is to control the heat extraction rate of the metal being cast. The term xe2x80x9cheat extraction rate,xe2x80x9d as used herein, refers to the rate of heat extraction from the molten metal in Watts. One way to control the heat extraction rate of the metal being cast is through cooling the mold surfaces in contact with the cast.
It can be difficult, however, to design a system for cooling a mold in a continuous caster because the mold is always in motion. Moreover, it can be difficult to control the complex, three-dimensional thermal loading of a mold. The cooling of mold surfaces should be carefully controlled to prevent undesirable thermal shocks and undesirable thermal loading of the mold from affecting the cast and causing unnecessary wear to the mold. Thermal shocks experienced by the mold as it cycles through the casting process and is repeatedly heated and cooled can cause fatigue stress resulting in premature wear of the mold, necessitating replacement. Moreover, undesirable thermal loading of the mold can cause residual heat to remain trapped in the mold. Residual heat remaining in the mold can prevent it from reaching its maximum heat extraction rate potential. Careful control of the mold cooling can reduce the formation of cold edge cracks in the cast. Careful control of the mold cooling can also prevent the formation of other imperfections that reduce the quality of a cast.
Several U.S. patents describe fluid cooling systems for use in continuous casters. For example, U.S. Pat. Nos. 4,934,444, by Frischknecht et al., and U.S. Pat. No. 3,570,583, by Lauener, both assigned to Lauener Engineering Ltd., disclose apparatus used in cooling molds of continuous casters. The apparatus consist of enclosures disposed in close relation to the molds, wherein cooling fluid is sprayed by nozzles to contact mold surfaces. The heated cooling fluid is collected in the enclosures and a vacuum atmosphere prevents cooling fluid from escaping from the enclosure. The mold surfaces can also be dried using forced air upon exiting the cooling enclosure.
U.S. Pat. No. 4,807,692, by Tsuchida et al., assigned to Ishikawajima-Harima Jukogyo Kabushiki Kaisha and Nippon Kokan Kabushiki Kaisha, discloses an apparatus for use in cooling the blocks of a continuous block caster. Tsuchida et al. disclose a cooling apparatus for blocks, wherein the blocks contain cavities which extend through their length in the direction transverse to the casting direction. A system of reciprocating nozzles aligned with the cavities in the blocks deliver cooling fluid to the blocks. The used cooling fluid is collected on the opposite side of the caster.
Known cooling systems typically use xe2x80x9cflushingxe2x80x9d processes for supplying cooling fluid to the heated mold surfaces. In a flushing process, large volumes of cooling fluid are brought into contact with the mold surfaces, typically by spraying the cooling fluid under pressure. Flushing processes alone, however are generally undesirable because such processes are difficult to control. For example, the cooling fluid can contain bubbles which contact the mold surface, creating uneven heat transfer across the mold/fluid interface. This can cause undesirable thermal shocking and undesirable thermal loading of the mold. Moreover, flushing systems are typically hand controlled and can be difficult to rapidly and repeatedly adjust in response to changes in the casting parameters, such as casting temperatures and cast quality, for example.
The present invention provides methods and apparatus for improving the quality of metal castings. The present invention provides methods and apparatus for cooling molten metal being cast in a continuous caster. The present invention provides methods and apparatus for controlling the thermal loading of a mold in a continuous caster. The present invention provides methods and apparatus which extend mold life in a continuous caster by reducing fatigue stress and premature wear of the surfaces of the mold. The present invention provides methods and apparatus for closed-loop control of the quality of metal being cast in a continuous caster.
In accordance with the present invention, apparatus are provided for cooling a mold used to solidify molten metal which utilize multiple cooling stages. Apparatus are also provided which allow control over the cooling of a movable mold in the casting direction (the xe2x80x9cx-directionxe2x80x9d) and the direction transverse to the casting direction (the xe2x80x9cy-directionxe2x80x9d).
In accordance with the present invention, apparatus are provided for measuring casting parameters for use in control of cooling, cleaning and coating of a mold in a continuous caster. Such casting parameters include mold temperatures, cast temperatures, melt temperatures, mold surface condition and cast quality.
In accordance with the present invention, apparatus are provided for cooling, cleaning and coating of a movable mold in a continuous caster. Mold cooling is preferably accomplished through contacting a thermally loaded mold surface with cooling fluid in droplet form. Such apparatus are capable of being automatically controlled to control cast quality without the need for human intervention.
In accordance with the present invention, methods are provided for use of the apparatus of the present invention. In particular, methods are provided for cooling, cleaning and coating a movable mold in a continuous caster. Moreover, methods are provided for controlling the cooling, cleaning and coating of a movable mold in a continuous caster. Such methods can be used for automatically controlling cast quality without the need for human intervention.